NUmERiCAL STUdY Of HELiCOPTER fUSELAgE...

PRACE iNSTYTUTU LOTNiCTWA ISSN 0509-6669215, s. 50-59, Warszawa 2011

NUmERiCAL STUdY Of HELiCOPTER fUSELAgE AEROdYNAmiCCHARACTERiSTiCS WiTH iNfLUENCE Of mAiN ROTOR

Jerzy Żółtak

WIeńczySłaW StaleWSkI

WIeSłaW zaleWSkI

Instytut lotnictwa

Abstract

The influence of simulated main rotor on aerodynamic properties of four different hel-

i copter fuselage configurations has been tested. The research was done using computa-

tional fluid dynamics (CFD) method. Simulations were done for hover and forward flight

conditions. The results for computation without and with main rotor modelling were

compared. Changes of aerodynamic properties with respect to basic configuration were

analysed.

1. introduction

In order to calculate properties of a helicopter, designers are often using aerody-namic characteristics of an isolated helicopter fuselage, obtained by calculation or ex-perimentally. Using this approach it is possible to determine the effect of the variousexternal elements integrated with fuselage [1] on its properties. this approach, how-ever, ignores the impact of the stream generated by the main rotor on fuselage loads.

the paper attempts to evaluate this phenomenon for a light helicopter, using themethods offered by modern cFD technique. Four configurations of helicopter fuselagewill be analysed, Figure 1:

· mB basic configuration [1] fuselage contains horizontal and vertical tails,· mB+2R configuration mB with modified (increased) horizontal tail,· mB+W basic configuration mB with additional listing elements (small wing)

mounted in central part of the fuselage,· md alternative geometry [2] of fuselage presented with horizontal and verti-

cal tailsthe md configuration was designed to improve aerodynamic parameters of mB con-

figuration, particularly to reduce negative lift force in forward flight condition [2]. InFigure 2 the basic differences between mB and md are shown.

the analysis will be focused on two aspects in following order:· influence of simulated main rotor on lift and drag forces and pitching moment

distribution on fuselage surface,· the relative change of forces and moment with respect to mB configuration.

51NUMerIcal StUDy oF HelIcopter FUSelage aeroDyNaMIc cHaracterIStIcS WItH INFlUeNce...

the discussion of the result will be preceded with short information about numeri-cal tools used for simulation and description of test conditions.

2. Numerical tests conditions

all numerical simulations were done using FlUeNt code [3]. It allows analysis ofsteady and unsteady flow field around any complex geometry. Motion of flow is de-scribed by the reynolds averaged Navier-Stokes equations. the finite volume methodis used to obtain numerical solution of these equations. Some turbulent models areimplemented in the software. a several type of boundary conditions can be used.

Figure 1. tested configurations of helicopter fuselage

In the presented analysis following parameters and settings were used:· 3 dimensional steady calculation· Spalart – allmaras turbulent model· external boundary condition – on all external faces of domain except the back-

ward one (outlet) a pressure far field boundary condition has been set; at out-let a pressure outlet with equal to far field pressure has been used,

· boundary condition at the fuselage – a wall boundary condition.For simulation of main rotor additional Virtual Blade Model [4] module was used.this module allows for relatively accurate modelling of the rotor influence on fluidavoiding modelling of complete blades, their geometry and movement.In the method Virtual Blade Model, the rotor is modelled as a layer of cells (Figure 3)with artificial sources of momentum of fluid. the momentum vectors are calculated

52 Jerzy Żółtak, WIeńczySłaW StaleWSkI, WIeSłaW zaleWSkI

based on the aerodynamic characteristics of the airfoils (sections of rotor blades), inaccordance with the Blade element theory.

Figure 2. comparison of the geometry of mB and md helicopter fuselage

the mesh for numerical simulation was generated using IceM cFD [5] software. thetypical mesh on fuselage surface and in main rotor domain is presented in Figure 3.

Figure 3. Helicopter fuselage surface and main rotor domain mesh distribution

3. Test conditions

the four configurations of helicopter fuselage presented above with and a configura-tion without main rotor influence were tested.In this calculation three-blade rotor based on IlS-2xx airfoils was modelled. the IlSairfoils aerodynamic characteristics database [6], [7], were used to obtain the mo-mentum produced by main rotor. In all simulation the same value of main rotorthrust was assumed. the tests were done for two fight conditions: maximum speedforward flight and hover.

4. Results

In order to make the obtained results more comparable, the geometry of each fuse-lage was divided into following sections, Figure 4:

· Front fuselage part· central fuselage part

· rear fuselage part· Vertical tail· Horizontal tail· Wing (only for mB+W configuration)

Figure 4. Fuselage’s section used in analysis.

For each case of simulation the aerodynamic forces and moments on the fuselage’ssection were obtained. Basing on these results, additional coefficients were definedfor particular analysis. First, to analyse of main rotor influence on aerodynamic forcesand moments distribution on the adequate fuselage’s sections, the following coeffi-cient was specified:

where Fs is force or moment calculated based on results of flow field analysed. By def-inition, the coefficient Qs saves the direction of aerodynamic force and sign of pitchingmoment.

the results for configurations: mB, mB+2R, mB+W and md, with and without sim-ulation of main rotor influence, were compared respectively in table 1, table 2, table3 and table 4.

In the tables the background of cells with positive value of coefficient has been setto gray. this way it is easier to notice, that the main rotor does not change the quali-tative image of forces and pitching moment coefficient distribution on analysed seg-ments of the fuselage.

Q Configuration tionF Configuration tion

F Confs

s

s

( ,sec )( ,sec )

(

iiguration total, )*100


table 1. comparison of forces and moment section distribution for mB configuration without and with simulation of main rotor

table 2. comparison of forces and moment section distribution for mB+2R configurationwithout and with simulation of main rotor


table 3. comparison of forces and moment section distribution for mB+W configurationwithout and with simulation of main rotor

table 4. comparison of forces and moment section distribution for md configuration without and with simulation of main rotor

For all configurations the total value of Qz for cases without and with main rotor isnegative. It denotes, that on fuselage the negative lift force is generated. a value oftotal QMy shows, that for configurations mB, mB+2R and mB+W pitching momentpushes down the fuselage’s nose. the situation for md configuration is different –pitching moment lifts up the nose of fuselage.

In next step the relative changes of drag and lift forces and pitching moment with re-spect to mB configuration is analysed. this analysis is limited to results of simulationwith modelling of working main rotor. In this case the following coefficient was de-fined:


where FS is calculated for analysed flow field drag, lift and pitching moment respec-tively.

table 5. comparison of lift force and pitching moment with simulation of main rotor for tested configurations in hover

table 6. comparison of lift force with simulation of main rotor for tested configuration for forward flight

From definition the positive value of defined above coefficient denotes in case of :· Δz that negative value of lift is reduced (positive phenomena)· ΔX that value of drag is increased (negative phenomena)· ΔMy that negative value of pitching moment is reduced

In table 5 the results of hover condition tests are collected. It can be seen that con-figurations mB+2R and md reduce the negative value of lift with respect to mB con-figuration. Quite different situation is observed for mB+W - the additional negativelift is generated on wing.

SS SConfiguration tionF Configuration tion F MB

( ,sec )( ,sec ) ( ,

ssec )

( , )*

tion

F MB totalS

100


table 7. comparison of force with simulation of main rotor for tested configuration for forward flight

the positive value of pitching moment, which characterizes the mB configuration, isreduced for both above configurations. For mB+2R this reduction is equal approxi-mately 75%. For md configuration reduction is so large, that radically changes theproperty of the fuselage. the fuselage’s nose moves down instead being lifted up.

the results for forward flight are presented in table 6, table 7 and table 8. In table5 the lift coefficients are compared. the influence of lift force generated on horizontaltail in mB+2R configuration (negative value) and wing in mB+W configuration (pos-itive value) on total value of coefficient Δz for this both configurations is easy to ob-serve.

Finally for mB+2R configuration the negative value of lift is decreased (value of liftincreases). lift is also increased by a little more than 50% for mB+W configuration.the reduction of negative lift is also noticed for md configuration, but it is smaller thancharacterized mB+W configuration. It is important to note that this reduction wasachieved by substantial redistribution of lift between fuselage components/sections:on central part of fuselage the negative value of lift increases, on front/nose part offuselage the significant reduction of negative lift is observable.

It is interesting how the above discussed changes in the lift force correlating withthe changes of the drag force. In table 7 the values of ΔX were collected. From thiscomparison one can see, that for mB+2R and mB+W increases of the drag, relate to mBconfiguration, are approximately 12 and 17 units respectively. the main part of dragincrement for mB+2R configuration is produced by horizontal tail. For mB+W con-figuration additional drag is generated by the wing. only for md configuration the re-duction of drag is observed - the value of ΔX decreases approximately 20 units. In thiscase, total value of ΔX is strongly dependent on two components: front (nose) and rearpart of fuselage. the contributions of both parts are approximately equal -24 units.

In table 8 the values of ΔMy for tested configuration are presented. For mB+W con-figuration the increase of pitching moment negative value is observed.


table 8. comparison of pitching moment with simulation of main rotor for tested configuration for forward flight

the pitching moment properties of both other configurations: mB+2R and md arecompletely different. they are characterized by positive value of pitching moment (thechange of ΔMy is greater than 100 units). Main influence on this change has the pitch-ing moment generated on horizontal tail (approximately 671 units) and front (nose)fuselage part (approximately 160 units) for mB+2R and md configuration respectively.

5. Conclusions

In the paper the influence of main rotor on aerodynamic parameters for different con-figurations has been tested. the research was done using computational fluid dynam-ics (cFD) method. the four geometries were briefly presented. First three (mB,mB+2R, mB+W) are having the same fuselage concept with different configurationsof external elements. the last fuselage (md) was numerically designed for the sameflight conditions as base configuration to improve its properties. Next the numericaltools (FlUeNt and VBM model) were shortly presented.at the beginning the influence of main rotor modelling on force and moment distri-bution on fuselage was tested. In this case the simulations for forward flight condi-tions were done for all fuselage configurations without and with main rotor. theresults of calculations show, that the main rotor does not change the qualitative imageof forces and pitching moment coefficient distribution on analysed components of allfuselage configuration.Next the change of forces and pitching moment of mB+2R, mB+W, md configurationwith respect of mB configuration properties was analysed. For this case calculationswere done for hover and forward flight conditions. Basing on results of these simula-tions the following conclusion can be formulated:

· For hover condition the configuration mB+2R and md reduces the negativevalue of force and changes the sign of pitching moment from positive to nega-tive, at the same time change is greater for the md configurations.



· the md+W configuration in hover has similar properties as mB configuration,but it achieves greater values of negative lift force and positive pitching mo-ment compared to base configuration.

· For forward flight for configurations mB+W and md the reduction of negativelift force can be observed

· only configuration md reduces the drag force· Similar as in hover, for forward flight the mB+r and md configurations, in re-

spect to mB configuration, changing the sign of pitching moment, but in thiscase the change is from negative to positive value.

above discussion leads to conclusion that md configuration of fuselage shows theway, how to change existing geometry in order to obtain better aerodynamic charac-teristics.

6. Bibliography

[1] grzeorczyk k., Dziubiński a., Numerical analysis the influence of the light heli-copter’s external components on the aerodynamic characteristics, transaction ofInstitute of the aviation (in print)

[2] Stalewski W., Żółtak J., Multi-criteria Design and optimisation of Helicopter Fuse-lage, in evolutionary and Deterministic Methods for Design, optimization and con-trol with applications Industrial and Societal problems, eds: poloni c., QuagliarellaD. at all, © cIMNe, Barcelona, Spain 2011 (in print)

[3] FlUeNt, http://www.ansys.com[4] http://www.fluvius.com.au/FlUeNt_UgM05/UgM05_Virtual_Blade_Model_VBM.p

df[5] IceM cFD, http://www.ansys.com[6] Instytut lotnictwa Internal report No. 115/Ba/95/D.[7] Instytut lotnictwa Internal report No. 117/Ba/95/D.

Acknowledgments

this research was supported by project No. o r00 0048 08 founded by Ministry of Sci-ence and Higher education.

Jerzy Żółtak. Wieńczysław Stalewski, Wiesław zalewski

NUmERYCzNE STAdiUm WPłYWU mOdELOWANiA WiRNikA NOśNEgO NA CHARAkTERYSTYki AEROdYNAmiCzNE kAdłUBA śmigłOWCA

Streszczenie

Wpływ symulacji wirnika głównego na własności aerodynamiczne czterech różnych

konfiguracji kadłuba śmigłowca został przebadany. Badania obliczeniowe przeprowad-

zono wykorzystując metody numerycznej mechaniki płynów. Obliczenia wykonano dla

zawisu oraz lotu postępowego. Porównano wyniki uzyskane bez i z symulacją wpływu

wirnika nośnego. Analizowano również zmianę własności aerodynamicznych badanych

konfiguracji względem konfiguracji bazowej.

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